1. Technical Field
This invention relates to novel fluororesin compositions, heat-shrinkable tubings and insulated wires employing such fluororesin compositions.
2. Description of Related Art
Fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and the like are known materials for heat-shrinkable tubings and coverings for insulated wires in those applications where highly heat-resistant properties are required.
The temperature ratings of PTFE and PFA referring to the UL index, are higher than 200.degree. C., so that these polymers show excellent heat-resistances. For instance, the changes of their mechanical properties such as tensile strength and elongation are very small even after the heat-aging in a Geer oven maintained at 260.degree. C. for 96 hours. In addition, heat-shrinkable tubings comprised of PTFE and PFA provide another advantage on account of their high transparency, in which the inside of the covered portion can be easily inspected when these heat-shrinkable tubings are used as a wire connection or joint.
However, PTFE and PFA are rather rigid, so that they tend to buckle when bent forcibly. Therefore the heat-shrinkable tubings and the insulated wires comprised of these polymers are not suitable for the narrow space wirings.
The radiation cross-linked fluorinated elastomers including known vinylidene fluoride based bipolymers or terpolymers such as vinylidene fluoride-hexafluoropropylene copolymer or vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer and the like, have good flexibility as well as excellent heat resistance, i.e., the operating temperatures higher than 200.degree. C. as similar to those of PTFE and PFA.
The heat-shrinkable tubings are generally manufactured as follows.
For instance, a tubular article which has been produced by an extruding method is cross-linked such as by ionizing irradiation. The cross-linked tubular article is radially expanded under a heated condition and then rapidly cooled to maintain its expanded shape. The heat-shrinkable tubings are required to maintain their expanded shapes before use, and this property is generally referred to as heat-settability.
However, the above-described cross-linked fluorinated elastomer tubings do not have enough heat-settability, i.e., ability to maintain their expanded shapes, so that those expanded fluorinated elastomer tubings shrink spontaneously at room temperature and lose their heat-shrinkabilities.
In order to improve the heat-settability of the fluorinated elastomer tubing, fluorinated polymers which have crystallinity such as a crystalline polyvinylidene fluoride are blended to the fluorinated elastomer. However this lowers the transparency of the tubing.
Furthermore, the fluorinated elastomers tend to tear against an external stress when they are applied as an insulation layer of an electrical wire.
Accordingly, it is an object of the present invention to provide novel fluororesin compositions.
It is another object of the present invention to provide a heat-shrinkable tube comprised of such fluororesin compositions which is highly tranparant and has excellent heat-resistance, flexibility and heat-settability.
Yet another object of the present invention is to provide an insulated wire incorporating an insulating covering or coating comprised of such fluororesin compositions which the covering or coating has excellent tear-resistance, heat-resistance and flexibility.